Optical lenses are typically produced from circular lens blanks which have a known front surface curvature, a known index of refraction, and a known diameter. The curvature of the back surface may be known, or it may be generated by various cutting assemblies known in the art. Once the appropriate back surface has either been selected or generated, then there is a further need to cut or edge the lens to match the shape of the frame into which the lens is to be placed.
A pair of eyeglasses has two edged lenses, each of which may have optical requirements unique to itself. The prescription written by the optician specifies the optical characteristics which each lens is to achieve upon being manufactured. The optician does not normally know the peripheral shape of the lens, and the lens maker must take the peripheral shape and other information into account when manufacturing the lens. It is required that the optical center of the lens be aligned with the pupil of the wearer, in order for the wearer to obtain the benefit of the eyeglasses. The optical center is that point where the prescription-derived optical characteristics are achieved, and the optical center should thus be aligned with the pupil of the eye. Modern eyeglasses, however, have lens shapes of a wide degree of variability, and it is unusual for the optical center of the lens to be aligned with the geometric center of the frame shape. The geometric center is that point where the horizontal distance (A) intersects the vertical distance (B) when the lens is boxed, i.e. placed within a square, as shown in FIG. 9, about the resulting lens. The geometric center may be both vertically and horizontally offset relative to the optical center.
Conventional edging technology calls for the lens to be rotated about its frame geometric center as the edging or grinding process proceeds. Because the optical center is not aligned with the geometric center, then it is important that the displacements therewith be taken into account when the lens is prepared for edging. If those displacements are not taken into account, then the optical center will not be properly positioned and the lens may need to be scrapped.
The lens blank may be adhesively secured to a circular device known as a block. The circular block may be plastic or some other material, and provides structure which the edging machine may grasp in order for the lens blank to be rotated. Various blocks are known in the art, as are various means for securing the blocks to the lens blank. One well-known blocking system is the 3M.RTM. adhesive blocker to which a circular brass block is applied.
Various devices are known in the art for assuring that the block is positioned on the geometric center of the frame shape cut-out. These devices typically require some manual manipulation by the lens maker, thereby potentially introducing human error into the positioning process. Furthermore, prior blockers introduce parallax error on account of the curvature of the lens surfaces, thereby creating another potential error. The manner in which the adhesive block is placed into contact with the lens also provides a potential for error.
Those skilled in the art will understand that there is a need for a lens blocker which assures proper positioning of the geometric center relative to the optical center in a manner which minimizes or eliminates the above potential positioning errors. The disclosed invention is one which meets those needs and avoids those errors because the optical center is positioned on a table system positionable in two axes relative to a fixed position block applicator. A computer control system calculates the geometric center and the displacements relative to the optical center, and outputs motor control signals in response thereto with the result that precise control over movement of the table system is provided.